Electrons surround the nucleus and can be removed by visible light (several eV) or higher energy photons (10 eV to kev), such as UV, X-ray (keV) or gamma ray(high keV to MeV). If one heats a gas hot enough, then the atomic collisions can cause electrons to be knock off atoms or molecules. The degree of ionization will depend on the density and temperature of the gas/plasma.
A particle kinetic energy of 1 eV corresponds to a temperature of ~11605 K, so 1 keV ~ 11.6 million K, and 1 MeV ~ 11.6 billion K.
Removing nucleons, i.e., protons and electrons, from the nucleus of an atom normally requires nuclear reactions involving nucleons, electrons, and even photons in the MeV range.